ABSTRACT. Smoking rates in Vietnam are one of the highest in the world. Promoting cessation is the key to reversing current global trends in tobacco-related mortality. Yet, there is a lack of smoking cessation interventions demonstrating efficacy in low middle-income countries (LMICs) like Vietnam. To increase smoking cessation rates and reduce smoking prevalence, programs need to be easily accessible, adapted to the local context and culture and reach large populations of smokers. Mobile phone interventions that use text messaging or short message systems (SMS) meet these criteria and are particularly well suited to address behaviors like smoking because messages can be tailored to individual needs and provide assistance in a smoker's natural environment. There is growing support for the efficacy of text-based smoking cessation interventions, but these studies have largely taken place in Western countries. The objective of this study is to develop and test the feasibility and acceptability of a smoking cessation SMS intervention adapted to the sociocultural context, language and communication styles of Vietnamese smokers. We propose a multi method three-stage approach to develop and test the feasibility, acceptability and preliminary effect on cessation outcomes of a SMS smoking cessation intervention among Vietnamese smokers. First, through surveys and focus groups we will linguistically and culturally adapt a text message library for Vietnamese smokers (AIM 1). Second, we will pretest of the SMS message intervention protocol with 40 subjects to examine, in real time using ecological momentary assessment, and post intervention qualitative interviews, preferences related to message content, frequency, and timing and make additional adaptations based on participant feedback (AIM 2). Finally, we will use the AIM 2 findings to refine the intervention and assess feasibility, acceptability and biochemically validated 3 month cessation rates in a two-arm randomized controlled pilot study comparing the SMS intervention with an assessment only control group (n=100) (AIM 3). We will also implement a program to build research capacity through mentorship, training and applied research experience. This project is significant in that it leverages the growth of mobile phone use in Vietnam to introduce an innovative and promising cessation intervention that has high potential for population level impact. Consistent with the purpose of the R21 funding mechanism, the expected outcomes of the project will provide a solid basis for a larger-scale trial testing a text message intervention informed by findings from this research.